Electron discharge device



Feb. 6, 1934. w KEARSLEY 1,946,288

ELECTRON DISCHARGE DEVICE Original Filed Sept. 19. 1929 Inventor:William K. Kearsley.

H is Att. ornqy.

Patented Feb. 6, 1934 1,946,288 ELECTRON DISCHARGE DEVICE William K.Kearsley,

Schenectady, N. Y., as-

signor to General Electric Company, a corporation of New York Originalapplication September 19, 1929, Serial No. 393,819. Divided and thisapplication May 12, 1932. Serial No. 610,866

5 Claims.

The present application is a division of my ap-' plication Serial No.393,819, filed September 19,

The present invention relates to the focussing of an electron beam by amagnetic field. It comprises in particular an arrangement whereby thefocussing of the electrons on the anode is automatically broadened orsharpened in accordance with the energy being expended so as not todamage the anode or other part upon which the electron beam impinges.

Heretofore, it has been common practice to so construct X-ray tubes sothat the electrons, or cathode rays, impinge on the anode, or target, ona spot of fixed area, known as the focal spot.

Only a certain amount of energy input can be safely used for a givensize of focal spot as otherwise the metal of the anode becomes fused oreven vaporized, thereby injuring or even destroying the X-ray tube. Tosecure sharp definition in radiographs the size of the focal spot mustbe small. When large X-ray output is desired some sacrifice indefinition is required as a larger focal spot necessarily must be usedto avoid overheating.

As the requirements of the radiographer in definition of radiographs andintensity of X-ray production, are variable, a well-equippedradiographer heretofore kept in stock ready for use several difierentkinds of X-ray tubes, some constructed to operate with a small focalspotand others with a larger focal spot.

As a consequence of my invention, I have provided an improved X-rayequipment whereby the size of the focal spot is automatically determinedso that with an increase in energy the size of the focal spot isincreased in corresponding degree and vice versa, thereby alwayssecuring for the use of the radiographer the most favorable focussingconditions.

. In accordance with the invention covered by the present divisionalapplication, the focussing of the beam of electrons is varied by amagnetic field which in turn is responsive to the flow of electricalenergy through the electron tube. As will be hereinafter more fullyexplained, an increase of electric energy flowing through the electrontube will result in an automatic broadening of the focus of the electronbeam and conversely a decrease of such energy will result ma sharpeningof such focus.

In the accompanying drawing, Fig. 1 is a diagram of an embodiment of myinvention which includes a thermionic X-ray tube which is provided witha magnetic focussing coil and Fig. 2

is an enlarged detail view showing in section the cathode portion of theX-ray tube shown in Fi 1.

In the system illustrated in Fig. 1, a magnetic focussing coil 3surrounds the X-ray tube 4 and is positioned about the path of theelectrons passing from the cathode to the anode to cause focussing ofthe space current flowing in the X-ray tube. The X-ray tube here shownis of the type described in United States Patent 1,203,495 W. D.Coolidge. The cathode 5 and the anode 6 of the X-ray tube 4 areconnected by the supply conductors 7, 8, to the secondary winding of atransformer 9, the primary winding of which is connected to the mains10, 11, by the conductors 12 containing a switch 13. The magnetic coil 3is connected to the secondary winding of the transformer 15, the primaryof which is connected to the mains 10, 11, in series with the variableresistor 16. Included in the supply circuit 17 of the winding 3 is thesecondary of a transformer 18. The primary of this transformer 18 isconnected in series with the secondary of the high potential supplytransformer 9 by the conductors 19 and therefore is traversed by currentflowing through the X-ray tube. The secondary of the transformer 18 isconnected in opposition with the secondary of the transformer 15 so thatan increase of current through the X-ray tube produces a decrease ofcurrent through the focussing coil 3. The cathode filament 5 of theX-ray tube is supplied by the conductors 20, 21 with heating currentfrom the secondary winding of a transformer 22 supplied from the mains10, 11 through a variable resistor 23.

The cathode of the X-ray tube, as shown in detail in Fig. 2 comprises aspiral filament 5, consisting preferably of tungsten, which is connectedat one end to a conductor 25 and at its opposite end to a ring 26carried by a conductor 27. The cathode conductors 25, 2'7 are sealedinto a stem 28 of glass or the like and by electric contact devices (notshown) are connected to the conductors 20, 21, Fig. 1. Surrounding thecathode filament 5 is a focussing device 29 consisting of a dishedtubular member which is supported by the conductors 30, 30'. These partsare mounted in turn on a split metal tube 31 which is carried by theglass tube 32. The latter with the stem 28 constitutes a reentrantmember in the cathode arm of the envelope of the X-ray tube 4.

When the current through the focussing coil 3 increases, the convergenceof the electron stream or cathode rays isincreased, or in other words,the focus of the X-ray tube is-sharpened. Conversely, a decrease ofcurrent through the coil 3 broadens the focus of the X-ray tube. In theoperation of the arrangement shown. in Fig. 1, when the current throughthe X-ray tube is increased, either by an increase of voltage of thesupply lines 10, 11, or if the value of the resistance 23 is decreased,thereby heating the cathode to a' higher temperature, the consequentincrease of current through the primary of the transformer 18 causes anincrease in the voltage opposing the voltage impressed upon thesecondary of the transformer 15 and produces a resultant decrease in thecurrent flowing through the supply circuit 17 and the coil 3. Thiswidens the focussing of the cathode rays and increases the focal area onthe anode. It is apparent that the coil 3 should be positionedlongitudinally with respect to the envelope of the X-ray tube to givethe best focussing operation throughout the entire range of currentsemployed in the X-ray tube.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. The combination of an X-ray device which is provided with athermionic cathode, means for producing a magnetic field about saidcathode for focussing an electrical discharge emanating therefrom, andmeans for automatically varying the strength of said field inversely inresponse to variations of current in said discharge device.

2. In combination, a thermionic discharge device, a coil for producing amagnetic field encircling the discharge path in said device, a maintransformer having a secondary winding connected to said dischargedevice, an energizing circuit for said coil and a second transformerhaving a primary winding connected in circuit with the secondary.winding of said main transformer and having a secondary windingconnected in series with the energizing circuit for said coil.

3. In combination, a vacuum tube having eleca in, a winding arranged toinduce a magnetic field for exerting a focussing effect on the dischargein said tube, a main electric supply circuit for said tube, an electricsupply circuit for said winding, and means in said supply circuit forsaid winding for varying the current in said winding inversely with thevalue of the current in said main supply circuit, whereby the focussingof said electron discharge is widened with an increase of current andsharpened with a decrease of current.

4. The combination of an X-ray tube which is provided with a thermioniccathode, an anode and an electrostatic focussing electrode surroundingsaid cathode, a magnetic coil encircling said tube about the path of anelectron discharge therein, main energy supply conductors connected tothe electrodes of said tube, auxiliary energy supply conductorsconnected to said coil and means responsive to the flow of current insaid main conductors for reducing the current in said auxiliaryconductors whereby the focal area on the anode of said tube is variedinversely with the value of the electron discharge therein.

5. An X-ray apparatus comprising the combination of an X-ray tube whichis provided with a thermionic cathode, a magnetic coil encircling thedischarge path of said tube and being arranged to exert a focussingeffect upon an electron discharge in said tube, an electric alternatwing current source connected to said X-ray tube, a transformer connectedto derive current from said source and having a secondary windingconnected to said coil, a second transformer also connected to derivecurrent from said source and having a secondary winding connected inopposition to the secondary winding of said first-mentioned transformer.

WILLIAM K. KEARSLEY.

